This invention relates to a device for the uniform irradiation of goods by means of electro-magnetic radiation having a quantum energy larger than 5 KeV.
Short-wave electro-magnetic radiation, as with X - or gamma rays, changes the physical, chemical and biological characteristics of materials. Technically, the radiation is applied in particular in the extinction of micro-organisms. Objects to be irradiated may include, for example, injection syringes, scalpels, suture material, feedstuffs, food for humans and enzymic systems. This is often done in packages ready for shipment. A certain minimum dosage practically ensures sterility. Further irradiation would usually only injure the goods to be irradiated. In the technically common sizes of recipients in which the goods are normally treated, the spreading of the radiation and its absorption are the reasons why not every volume element receives the same dosage of irradiation. Even if, according to the state of the art, the recipients are being irradiated from several sides, considerable partial overdosages will be required to reach the minimum dosage necessary, e.g. for sterility. With some of the widely used plastics this leads to a reduction of material characteristics, as e.g. the increased embrittlement of polypropylen or the decoloration and smell development with polyvinylchloride. Furthermore, the overdosage leads also to a quality decrease in foods or to a considerable reduction for the biological effectiveness of enzymic systems. In the following, the relationship of largest dosage to smallest dosage in a certain material to be irradiated, e.g. irradiation containers like a cardboard box or a barrel, is called "overdosing ratio". Uniform irradiation means that the overdosing ratio will be close to 1.
There is a device known from the ATOMIC ENERGY OF CANADA LIMITED (in the following called "device A"), in which a number of cardboard boxes containing the goods to be irradiated and placed horizontally in two layers move around a .sup.60 Co radiation source in the shape of a 1 m.sup.2 sized table in such a way that the cardboard boxes are radiated from two opposite sides. The cardboard boxes have to be turned after one half of the envisaged irradiation time has passed or the position of the track has to be changed. The overdosing ratio rises, e.g. for cardboard boxes of a size of 55.2 cm.times. 43.2 cm.times. 91.4 cm and a medium filling density (or packing density) of 0.3 g/cm.sup.3, to values already too high for many applications.
Another device of the RADIATION DYNAMICS LIMITED (in the following called "device R") operates on the principle that six containers rotate on a circular path of 2.2 m in diameter around a .sup.60 Co rod-type source of approximately 45 cm length which moves up and down. The ashlar-type containers have each a square base and turn in the same direction around their axes. Rotation takes place in steps of 90.degree., so that practically one flat side is always directed towards the radiation source. Thus, the overdosing ratio with a given container size of for example 70 cm.times. 70 cm.times. 250 cm and a filling density of 0.7 g/cm.sup.3 is larger than 2.
There are devices known for application in research (e.g. from ATOMIC ENERGY OF CANADA LIMITED) which have a number of rod-shaped radiation sources arranged in the form of a circle or use a X-ray cup anode. Apart from the volumes which are not uniformly irradiated at the axial ends, a cylindrical inner space is obtained in the center which has everywhere an almost constant dosage capacity. However, as soon as the goods to be irradiated are introduced into this inner space, the dosage capacity decreases considerably, especially in the case of objects of high density. This fact can be a great obstacle to research work.
The German patent specification 1.953,135 describes a device which operates on the basis of a radiation source having the shape of a table. The incorporation of a fadeout device consisting of prismatic rods leads to an improvement, as far as the decrease in dosage capacity with increasing distance normal to the radiation source is concerned. Otherwise this result is obtained only by means of a considerably larger plate-shaped radiation source. However, a somewhat more uniform irradiation always means that a reduced radiation efficiency has to be put up with, due to the prismatic rods.
However, the problem of producing a really uniform irradiation dosage in the goods to be irradiated, i.e., in matter, cannot be solved by means of creating a better and more homogeneous field of radiation in air-- the common feature of the described known devices-- not even by means of irradiation from several sides. The reason for this is that of the two values which influence the uniformity of the irradiation namely, the spreading of the radiation and the absorption of radiation, only the spreading of radiation is influenced. Thus the effect of the absorption of radiation which leads to the phenomenon that a dosage minimum occurs in the center of the goods to be irradiated, can in no case be compensated.
With the device according to the invention it is not endeavoured to create a homogeneous field of radiation in air. On the contrary, the field of radiation is intentionally considerably distorted, i.e., it is made more inhomogeneous. Surprisingly, it is thereby possible to compensate for the influence of the absorption of radiation. Thus, the goods to be irradiated are receiving the same radiation dosage in each volume element, which means there will be no dosage minimum in the center.